Anycast will pull in traffic to the closest (hop distance) datacenter for a client, which won't be the right datacenter a lot of the time if everything lives under one domain. In that case they will have to route it over their backbone or re-egress it over the internet, which does cost them money.
https://twitter.com/colmmacc/status/1067265693681311744
Google Cloud took a different approach based on their existing GFE infrastructure. It does not really seem to have worked out, there have been a couple of global outages due to bad changes to this single point of failure, and they introduced a cheaper networking tier that is more like AWS.
I don't think that's true. Route53 has been using Anycast since its inception [0].
The Twitter thread you linked simply points out that fault isolation is tricky with Anycast, and so I am not sure how you arrived at the conclusion that you did.
[0] https://aws.amazon.com/blogs/architecture/a-case-study-in-gl...
It's a good choice for DNS because DNS i a single point of failure anyway, see yesterdays multi hour Azure/Microsoft outage!
Their DNS nameservers which resolve those subdomains do of course.
S3 isn't designed to be super low latency. It doesn't need to be the closest distance to client - all that would do is cost AWS more to handle the traffic. (Since the actual content only lives in specific regions.)